1. Field of the Invention
This invention relates to optically coupled drivers and more specifically, to a circuit that reduces the average light emitting diode current required to latch the output device.
2. Prior Art
Requirements exist wherein data is transmitted between two circuits that must be electrically isolated from each other. For example, a microprocessor that switches on a high voltage motor may require isolation to prevent noise from the motor from interfering with the circuitry of the microprocessor, and any high voltage device switched on or off by an operator may require isolation of the manually operated switch as a safety precaution. In the past, such isolation was accomplished with devices such as relays and isolation transformers. Recently, however, the isolation has been accomplished, more cheaply and efficiently, with an optical coupler using semiconductor technology. A typical optical coupler has a light emitting diode and a triac mounted in a single package, but physically separated. An input signal drives the light emitting diode causing it to emit light which passes through a medium to the triac. The light striking the triac turns it on, providing a signal that is used to supply a current to the load. Typical medium resistance between the input light emitting diode and the output triac may be 10.sup.11 ohms.
One optical coupled triac driver presently being manufactured is enclosed in an integrated circuit package having six terminals. The light emitting diode is coupled to two input terminals for receiving a DC voltage input signal. These present devices require that the light emitting diode be driven with DC current at a relatively high current, e.g. 15 milliamperes, to latch the output triac. This high current is a problem in some applications.
Thus, the need exists for an inexpensive circuit for driving a light emitting diode of an optically coupled driver that operates at a lower average current.